An uninterruptible power supply (UPS) is a power supply apparatus for providing a load with uninterrupted, high-quality, reliable AC power, and having functions of protecting and monitoring the state of power supply in real time, which plays an important role in improving quality of power supply and ensuring normal operation of an apparatus. From perspectives of the circuit structure and mode of uninterrupted power supply, the UPS is mainly divided into two categories of a back-up type and an online type.
FIG. 1 exemplarily shows a configuration block diagram of a conventional online UPS. When the power of the grid is normal, an input AC voltage is converted into a DC voltage after rectified and filtered, to directly drive an inverter to power a load, while charging the battery. The inverter outputs an AC output voltage of regulated voltage and frequency to the load. When the power of the grid is abnormal or interrupted, the inverter converts the DC voltage supplied by the battery into the AC voltage, and supplies it to the load, so as to achieve uninterrupted power supply.
Based on the concept of the online UPS, a variety of circuit topologies and control methods have been developed. A topology of the conventional online UPS is shown in FIG. 2, which comprises two-stage power conversion, namely, a power factor correction (PFC) stage and an inverter stage. Due to presence of the two-stage power conversion, limited by the existing semiconductor and magnetic material technology, it is difficult to further improve the overall conversion efficiency. For example, if both power conversion stages are optimized in order to obtain an efficiency of 98%, the overall efficiency will drop to about 96%. Although such UPS topology comprises a highly-efficient bypass mode of opening a bypass switch and closing a power conversion stage, yet operation conditions of this mode are limited by many factors, which is too dependent on the quality of the input power of the grid.
There is still room for improving the online UPS.